1. Field of the Invention
The present invention relates to a transponder unit for transferring data to a reading device by means of an electromagnetic field, to a system comprising the reading device and the transponder unit, and to a method for transferring data from the transponder unit to the reading device.
2. Related Art
Transponder units are used for wireless data communication in particular in the near zone. Known methods for data transfer in the near zone are for example Bluetooth, WLAN, DECT, RFID or NFC.
RFID (Radio Frequency Identification) is a data transfer method by means of electromagnetic fields in a frequency range of e.g. 13.56 MHz or 868 MHz. The RFID transfer method offers the possibility of taking from the field of the reading device the energy required for operation of the transponder by means of the antenna employed at the same time for data transfer. Hence, the RFID transfer method is particularly suitable for chip cards involving contactless data transfer.
NFC (Near Field. Communication) is a data transfer method by means of electromagnetic fields in the frequency range of 13.56 MHz. The methods used for data transfer in NFC are very similar to those of contactless RFID chip cards. NFC devices can both communicate with RFID chip cards and simulate RFID chip cards.
In RFID and NFC the data transfer from the transponder unit (or the NFC device acting as a transponder) to the reading device (or the NFC device acting as a reading device) can be effected in particular through load modulation or by means of a method for modulating the backscatter cross section (electromagnetic backscatter coupling).
Upon data transfer from the transponder unit to the reading device through load modulation (inductive coupling), the alternating current which is induced in the transponder coil by an electromagnetic field generated by the reading device is influenced by switching a load resistor on and off. According to the principle of transformer coupling, this effects a change of current and voltage on a primary coil of the reading device with which coil the electromagnetic field is generated. This makes it possible to transfer data.
Upon data transfer from the transponder unit to the reading device through modulation of backscatter cross section, the backscatter cross section, which gives information about how strongly an object reflects electromagnetic waves, is modulated. For example, antennas have an especially strong backscatter cross section in case of resonance.
From the patent application DE 10 2004 031 092 A1 there is known a transponder unit which can communicate with a commercially usual RFID reading device conceived for data reception by means of load modulation or modulation of backscatter cross section, or with an NFC device acting as a reading device. Said transponder unit possesses not only a separate energy supply, but is also configured as an actively transmitting transponder unit. The actively transmitting transponder unit sends data to a reading device by means of a self-generated field, in such a way that the generated signal differs for the reading device only slightly from a modulation as would be generated by a conventional transponder unit or an NFC transmitter. Without a load modulation or backscatter modulation of the reading device field generated by the reading device, the transponder unit thus itself generates a field which simulates to the reading device a modulation of the reading device field by a transponder unit. By means of this method the range can be increased. An application of this method in (small) memory cards, such as a multimedia card or a μSD card, in particular in combination with a secure element, is described in the patent application DE 10 2005 061 660 A1.
In conventional RFID/NFC systems, the electromagnetic field generated by the reading device can be continually received via an antenna of the transponder. The frequency of the RF alternating voltage induced in the transponder antenna can thus be continually evaluated and is thus available to the transponder as a clock frequency for clocking the data transfer between transponder and reading device. The clock frequency then corresponds exactly to the transmitting frequency of the reading device and is coupled thereto with phase synchronization. It is of no matter here whether the transponder unit is receiving data from the reading device, is in a non-communication mode (e.g. processing a command, waiting for a command) or is sending data to the reading device e.g. by load modulation or by means of modified backscatter cross section.
In contrast, in the method described by the above-mentioned patent applications DE 10 2004 031 092 A1 and DE 10 2005 061 660 A1, in which method the transponder unit sends data to the reading device in an active transmit mode, the clock signal of the reading device is superimposed, during the transmit mode of the transponder unit, by the signal emitted by the transponder unit and can no longer be received by the transponder unit. Hence, during the transfer of data such an actively transmitting transponder unit requires its own frequency generator (e.g. a quartz oscillator).
There can thereby occur a frequency difference between the clock frequency of the reading device and the frequency generator frequency of the frequency generator of the active transponder. This subsequently leads to a slight deviation between the bitrate expected by the reading device and the actual bitrate of the data sent by the transponder unit, and can lead to errors in data transfer when data bits are sent at a different time point from that expected by the reading device.
To avoid this problem, DE 10 2004 031 092 A1 proposes synchronizing the frequency generator with the clock signal emitted by the reading device in phase lock by means of a phase-locked loop (PLL) circuit. However, such a circuit increases the production cost of the transponder unit.
Hence, it is the object of the present invention to provide an actively transmitting transponder unit which guarantees a reliable data transfer with simple means.
This object is achieved by the features of the independent claims. In claims dependent thereon there are stated advantageous embodiments and developments of the invention.